I'm fairly sure it's Coprinus comatus (https://en.wikipedia.org/wiki/Coprinus_comatus), lawyer's wig or shaggy ink cap. But fungi are notoriously difficult to identify - it takes an expert to be sure, and even then not always.
A few days ago I saw field mushrooms, I think, but left them alone, partly because they were pretty ratty, partly because of the amount of dog dirt, mainly because I'm just not confident enough to rely on my identifications. It's a case of don't do this at home ...
On a different subject, we now know when the Fair's coming to town: